Britain's King Charles III sounded "like a lottery winner" after the death of his late first wife Diana, her brother writes in his forthcoming book, new extracts of which were published on Thursday.

Charles Spencer said the then-Prince Charles was "giddily elated" in the immediate aftermath of Diana's death in 1997, possibly because he felt it meant he was freer to marry again, according to passages in the Daily Mail.

The King on Wednesday took the unusual step of responding to criticism after the first extracts of Spencer's book were published in newspaper, saying via a spokesperson that grief could impact reason, judgement and memory.

There was no new comment from Charles on the latest excerpts, which also stated that Diana went to a site of many suicides on Britain's south coast, but that her love for her sons, Princes William and Harry, "stopped her from going further".

"It is a sad fact that, in her later years, Diana drove herself more than once to Beachy Head, the highest chalk sea cliff in England, while contemplating ending it all," Spencer wrote.

Earl Charles Spencer. Photo: Getty Images for Invictus Games Foundation

Diana was killed at the age of 36 when the car carrying her and her lover Dodi al-Fayed crashed in a Paris tunnel as it sped away from chasing paparazzi photographers on motorbikes.

Spencer says in the book that he spoke to Charles shortly after Diana's death was confirmed, writing: "Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he sounded giddily elated – like a lottery winner, I remember thinking at the time."

Spencer added: "Looking back now, I suspect the prince's first reaction must have been to think more of how this event might free him to marry the woman he loved, rather than appreciating the loss of one he did not."

Charles and Diana separated in 1992 and divorced in 1996.

That was a year after Diana told the BBC that "there were three of us in this marriage, so it was a bit crowded" in reference to Charles' relationship with Camilla Parker Bowles, whom he married in 2005.

A Buckingham Palace spokesperson said on Wednesday: "While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgement and colour memory in ways others do not recognise, even many years after such a loss."